1. Field of the Invention
The present invention relates to an image forming apparatus suitable as a copying machine, a facsimile machine or a printer.
2. Description of the Related Art
Heretofore, there has been known an image forming apparatus as disclosed in Japanese Patent Laid-Open Publication No. 04-60660 (hereinafter referred to as “document D1”). This image forming apparatus is designed to apply a bias voltage through a charge roller to an outer peripheral surface of a photosensitive drum which is being rotated about a given drum shaft so as to electrostatically charge the outer peripheral surface uniformly. Then, a laser beam generated based on image information is emitted onto the uniformly-charged outer peripheral surface of the photosensitive drum to eliminate a part of the uniformly charged area so as to form an electrostatic latent image. Toner particles are then supplied to the electrostatic latent image to form a toner image. This toner image on the outer peripheral surface of the photosensitive drum is electrostatically attracted toward a sheet having charges of opposite polarity formed using a transfer roller, and transferred onto the sheet. After the transfer process, residual toner particles on the outer peripheral surface is removed by a given cleaning device. Then, for the next image forming job, the outer peripheral surface is subjected to a charge process based on the charge roller.
In the above type of image forming apparatus, the document D1 discloses a brush roller provided as a means to clean an outer peripheral surface of the charge roller. The brush roller is designed to be rotated in such a manner that the outer peripheral surface of the charge roller is slidingly rubbed with and cleaned by bristles on an outer peripheral surface of the brush roller. According to this technique, even if residual toner particles and fine additive particles which have not been able to be removed from the outer peripheral surface of the photosensitive drum by the cleaning device are attached on the outer peripheral surface of the charge roller, these undesirable substances will be removed by the brush roller. This makes it possible to prevent the outer peripheral surface of the photosensitive drum from being nonuniformly charged so as to avoid deterioration in image quality due to charge nonuniformity.
In the process of transferring a toner image on the photosensitive drum to a sheet, electric charges having opposite polarity to that of electric charges of the toner image are imparted to the sheet, and the toner image formed on the outer peripheral surface of the photosensitive drum is electrostatically taken away by the opposite-polarity charges, and transferred to the sheet. Consequently, the opposite-polarity charges are applied to the outer peripheral surface of the photosensitive drum to form a so-called “transfer memory”.
Even using the technique disclosed in the document D1, the charge roller cannot remove such a transfer memory, and a problem about charge nonuniformity occurring in the outer peripheral surface of the photosensitive drum still remains.
An image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 06-83249 (hereinafter referred to as “document D2”) can be taken as one example of measures for solving the above problem. The image forming apparatus disclosed in the document D2 comprises an exposure lamp provided as a charge erasing device and disposed on an upstream side of a charge roller (primary charge roller) designed to be applied with a DC voltage or a superimposed voltage of DC and AC voltages, and an additional charge roller (preliminary charge roller) disposed on an upstream side of the exposure lamp. The document D2 discloses a technique of applying a DC voltage, or a superimposed voltage, having the same polarity and value as those of the voltage in the primary charge roller, to an outer peripheral surface of a photosensitive drum by the preliminary charge roller, erasing electric charges by the exposure lamp, and then imparting electric charges to the outer peripheral surface of the photosensitive drum by the primary charge roller.
According to the technique disclosed in the document D2, the preliminary charge roller is operable to counteract opposite-polarity charges on the photosensitive drum, and the charge erasing device is operable to erase the charges. Then, the primary charge roller is operable to impart given electric charges to the outer peripheral surface of the photosensitive drum. This makes it possible to effectively prevent charge nonuniformity in the outer peripheral surface of the photosensitive drum.
In the image forming apparatus disclosed in the document D2, if toner particles and/or an extraneous substance (magnetic fine particles as an additive to be attached onto surfaces of toner particles, etc.) attached on the outer peripheral surface of the photosensitive drum are not fully removed therefrom even though the opposite-polarity charges on the outer peripheral surface of the photosensitive drum are erased by the preliminary charge roller, these undesirable substances will be attached onto the primary charge roller. Consequently, electrical characteristics of an outer peripheral surface of the primary charge roller become uneven to cause difficulty in uniformly charging the outer peripheral surface of the photosensitive drum. Thus, a problem about inability to reliably eliminate charge nonuniformity in the outer peripheral surface of the photosensitive drum still remains.